For a high frequency application, an amplifier circuit amplifying a signal is essential. In order to increase amplification (a gain) of an amplifier circuit, there has been employed a method in which unit amplifier circuits are cascade-connected. Normally, there have been employed methods in which grounded-source transistors are cascade-connected and a grounded-source transistor and a grounded-gate transistor are cascade-connected. In the above case, there is a need to match (for matching) each of the transistors, and it is difficult to obtain high amplification over a broad band. To describe concretely, an input of a grounded-source amplifier circuit is capacitive, and there is often a case that inductors are used in series in order to match the input to an input impedance, (which is 50Ω normally). The input is well matched in the vicinity of a center frequency of a matching circuit as above, and thus a gain is large, but at a frequency away from the center frequency, a gain is reduced due to mismatching. As described above, the amplifier circuit using a grounded-source transistor at an input stage has a difficulty in obtaining amplification over wide frequencies.
There has been known a circuit in which a feedback circuit composed of a resistance and a capacitance is disposed between an input and an output of a circuit in which grounded-gate transistors are connected in two stages. The feedback circuit is interposed between the input and the output to thereby reduce input and output impedances over a broad band. It is not possible for the above circuit to fulfill a purpose with regard to the point of increasing a gain. Next, a reason thereof is described. In the above circuit, the input impedance of the grounded-gate amplifier circuit expressed as 1/gm has to be set higher than an impedance of the feedback circuit. Concretely, it is necessary to make gm small by using transistors with small gate widths. Eventually, since gm is made small, a voltage gain is reduced.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2008-141475